1. Field of the Invention
This invention relates generally to a means for current transfer, and more particularly, to a means for high current transfer using a Bi-Cd-In-Sn or Bi-Cd-In-Sn-Pb alloy.
2. Description of the Prior Art
High-current-density electrical machinery has extraordinary current collector requirements. Superconducting homopolar motors, for example, which operate at extremely high current densities (approximately 10,000 A/in.sup.2 (15.5 MA/m.sup.2)) require electrical current collector components capable of handling very large current loads with a minimum of current losses at the collection interface. Solid state brushes, such as graphite and graphite composites, commonly used in conventional motors and generators, are not feasible for use in high-current-density or superconducting motors or generators because of high wear rates and limited current carrying capability. Liquid metals or alloys which are liquid at the operating temperatures of the systems in which they are used, on the other hand, have proved to be viable alternative current collector materials in such machinery.
Ideal liquid metals or alloys for such use would have low melting points, low densities, high thermal stability, high physical stability, high ability to wet the current collector surfaces in machinery in which they are used, low reactivity with oxygen, and low toxicity. In superconducting homopolar motors the rotational forces are such that sufficient liquid metal density is required to retain the alloy in motor channels. High alloy stability under the rotational forces involved would also be a valued characteristic.
Heretofore, mercury, gallium alloys, and a liquid metal eutectic of sodium and potassium containing 78 weight percent potassium and 22 weight percent sodium (NaK-78) have been the materials of choice for use as electric brush material in high current density systems. While these metals generally have low melting points, low densities, and varying ability to wet the current collector surfaces of machinery in which they are used, each has serious shortcomings, NaK-78, like all alkali metals, is highly reactive with oxygen and violently reactive with water. In addition, NaK-78 causes severe burns upon contact with the skin. These properties dictate a need for sophisticated handling procedures and maintenance of a very high purity cover-gas environment. Further, during use in superconducting motors, NaK-78 displays a tendency to migrate from the collector sites and proves unable to wet copper under operating conditions. Likewise, mercury and gallium alloys, both of which have been used in land-based superconducting motors, possess shortcomings in that they are toxic, prone to cause corrosion problems, and unstable under the rotational forces involved in superconducting machinery.
Fusible alloys; usually the binary, ternary, quaternary, and quinternary mixtures of bismuth, lead, tin, cadmium, and indium; are well known in the alloy art for applications where low melting point is a desired property. Examples include Rose's Alloy (Bi 50 weight %, Pb 28 weight %, Sn 22 weight %), Wood's Metal (Bi 50 weight %, Pb 25 weight %, Sn 12.5 weight %, Cd 12.5 weight %), and those alloys provided in U.S. Pat. Nos. 4,083,718 and 4,214,903. Many compositions of eutectic fusible alloys, which have definite and minimum melting points as compared with other compositions of the same metals, are also well known in the alloy art. Although these alloys have numerous known uses, the prior art does not teach composition of a fusible alloy especially suitable for use as a current transport material in high-current-density electrical systems nor does it teach such use of a fusible alloy.